1. Field of the invention
The invention relates to equipment for the guidance of a moving web and more particularly to an apparatus which changes the direction of the web's advance without contacting the web.
2. Description of the Related Art
In U.S. Pat. No. 4,760,946 a device is described for changing the direction of a web, without contacting the concave web surface formed at the inner side of the curved web section. The working principle disclosed is based on applying a differential pressure between the concave inner surface and the convex outer surface of the web which assumes a cylindrical shape at this region. The pressure differential is created with the help of a housing connected to a source of reduced pressure, which cooperates with a surface of a support to effect the direction change. The area between the web and the housing is identified as an outer zone and the area between the web and the back-up surface is identified as an inner zone. The inner zone communicates with the atmosphere. To seal the outer zone from the atmosphere and maintain a reduced pressure, sealing elements extending across the total width of the web at the points where the web enters and exits the housing are provided.
The embodiments illustrated in U.S. Pat. No. 4,760,946 use rollers as sealing elements, each of which forms a first sealing point with the web and a second sealing point with a wall of the housing. This configuration allows a very efficient sealing of the housing against the atmosphere because no air leakage can occur between the roller and the web in contact with it.
Also the efficiency of sealing at the position between the roller and the housing can be improved significantly by designing this clearance as a long and narrow gap. This capability results in a number of advantages with respect to design flexibility for the housing and the reduced pressure source.
The major disadvantage of the use of sealing rollers, however, is the inevitable contact of the outer web surface by the rollers. This excludes the use of such a design in combination with materials tolerating no contact on any of the two web surfaces, as is the case for web materials freshly coated or printed on both sides. The same problems arise when scratches or electrostatic charging and subsequent electrostatic discharge marks are to be avoided.
Non contacting web turning devices have been developed, but such devices generally suffer from web instability problems particularly serious at the starting and ending points of the web direction change.
There is still a need for an apparatus and method which can alter the web direction without any contact with the outer web surface in a way which prevents web instabilities particularly at the beginning and ending points of the web direction change.